This invention relates to systems and methods of manufacturing single mode optical fiber. More particularly, it relates to defining spin functions for reducing PMD over a broad band of fiber birefringence while minimizing twist introduced on the optical fiber. This application is being filed concurrently with application Ser. No. 10/202,560, entitled System And Method For Obtaining Spin And Mechanical Twist Data During Optical Fiber Draw, which is incorporated by reference into this application.
It is well known that the so-called “single mode fiber” that is commonly used in communication systems is not purely single mode. Rather, two modes, with perpendicular polarizations, exist in single mode fiber. See, for example, Dandliker, R., Anisotropic and Nonlinear Optical Waveguides, C. G. Someda and G. Stegeman (editors), Elsevier, N.Y., 39-76, 1992. Mathematically, these two polarizations form an orthogonal basis set. Accordingly, any configuration of light that propagates through a single mode fiber can be represented by a linear superposition of these two modes.
If the fiber is perfectly circularly symmetric in both geometry and internal and applied stress, the two polarization modes are degenerate. The modes would propagate with the same group velocity and have no time delay difference after traveling the same distance in the fiber. However, a typical optical fiber is not perfectly circularly symmetric. Imperfections, such as geometric and form deformation and stress asymmetry, break the degeneracy of the two modes. See, for example, Rashleigh, S. C., Journal of Lightwave Technology, LT-1:312-331, 1983. As a result, the two polarization modes propagate with different propagation constants (β1 and β2). The difference between the propagation constants is termed birefringence (Δβ) and is expressed as:Δβ=β1−β2Birefringence causes the polarization state of light propagating in the fiber to evolve periodically along the length of the fiber. The distance required for the polarization to return to its original state is the fiber beat length (Lb), which is inversely proportional to the fiber birefringence. In particular, the beat length Lb is given by:Lb=2π/ΔβAccordingly, fibers with more birefringence have shorter beat lengths and vice versa. Typical beat lengths observed in practice range from as short as 2-3 millimeters (a high birefringence fiber) to as long as 10-50 meters (a low birefringence fiber).
In addition to causing periodic changes in the polarization state of light traveling in a fiber, the presence of birefringence means that the two polarization modes travel at different group velocities; the difference increasing as the birefringence increases. The differential time delay between the two polarization modes is called polarization mode dispersion, or PMD. PMD causes signal distortion that is very harmful for high bit rate systems and analog communication systems.
Various methods to reduce PMD have been disclosed. One prior art method of reducing PMD involves spinning the preform (the pure glass form which the fiber is formed), during the fiber drawing process. See, for example, Barlow, et al., Applied Optics, 20:2962-2968, 1981; Payne, et al., IEEE Journal of Quantum Electronics, QE-18:477-487, 1982; Rashleigh, “Fabrication of Circularly Birefringent Single Mode Fibers,” Navy Technical Disclosure Bulletin 5:7-12, 1980; and PCT Patent Publication No. WO 83/00232. Spinning causes the internal geometric and/or stress asymmetries of the fiber to rotate about the fiber's axis as one progresses down that axis. By performing the spinning during drawing, i.e., when the root of the preform is substantially molten, essentially pure rotation is performed on the fiber asymmetries, as opposed to a combination of rotation of the asymmetries and the introduction of rotational stress as would occur if the fiber were twisted after having been drawn. For a discussion of the use of spin to reduce PMD see, for example, Schuh et al., Electronics Letters, 31:1172-1173, 1995; and Ulrich, et al., Applied Optics, 18:2241-2251, 1979.
Another method of reducing PMD is disclosed in U.S. Pat. No. 5,298,047 to Arthur C. Hart, Jr. et al., (hereafter “Hart”), which discusses reducing PMD by a relatively low rate spinning of a fiber, as opposed to a preform, during the drawing process. (See also U.S. Pat. No. 5,418,881). More particularly, the Hart patent discloses a spin function which varies in a substantially sinusoidal manner. That is, Hart's spin rate α as a function of distance z along the length of Hart's fiber can be written as:α(z)=α0 sin(2πƒz)where α0 is Hart's spin amplitude in turns/meter and ƒ is Hart's longitudinal frequency in inverse meters, i.e., ƒ represents the rate at which Hart's spin rate α varies along the length of the fiber.
The term “spin function” as used herein describes the spin rate as a function of distance z, i.e., α(z), or as a function of time t, i.e., α(t). The time spin function applied to a fiber is directly derivable from the corresponding distance spin function through the fiber draw rate (and vice versa). The draw rate is normally constant in the general case, but can be variable. As discussed more fully below, the spin function employed in producing a fiber, whether expressed as a function of distance or expressed as a function of time, and the resulting spin function present in the finished fiber, are not generally identical. One reason for the difference is because of mechanical effects in the equipment handling the fiber, e.g., slippage at the interface between the fiber and the apparatus used to apply the spin function to the fiber and/or preform.
U.S. Pat. No. 5,943,466 to Henderson (hereinafter “Henderson”) discloses improved spin functions that are: (1) not substantially constant, i.e., they change substantially as a function of distance along the length of a fiber or as a function of time; (2) not substantially sinusoidal; and (3) have sufficient variability, e.g., sufficient harmonic content, to provide a substantial reduction in PMD for a plurality of beat lengths.
Henderson discloses a variety of non-uniform spin functions. For example, a spin function can be constructed as a weighted sum of sinusoidal components of different frequencies with the number of components and their weights being chosen to produce an overall function that achieves the PMD reductions of the invention. The spin function can also be randomly generated. In certain preferred embodiments, the spin function is a frequency-modulated or an amplitude-modulated sinusoidal function, the modulation being sufficient to cause the spin function to not be substantially sinusoidal.
Regardless of which method is used, a spin is imparted onto the optical fiber and the nature of the spin imparted impacts the degree to which PMD is reduced. A spin is “impressed” on the fiber when the fiber in the hot zone and is caused to be torsionally deformed resulting in the deformation being ‘frozen’ into the fiber as it cools from its molten state. Once cooled, the fiber exhibits a permanent “spin”, i.e., a permanent torsional deformation. Importantly, however, the amount of spin that is actually introduced into the molten fiber is not always the same as the amount that is attempted to be introduced. There are various factors effecting the rotational transfer.
For example, FIG. 1 illustrates an aparatus for forming optical fibers. The optical fiber 25 may ‘slip’ on the rollers imparting the twist 60. Further, the length of fiber span between the molten fiber and the spinning apparatus 60 impacts the degree of cooling and thus the amount of spin actually impacted. The spinning apparatus 60 may comprise a roller 191 or other means to provide spin and imparts an angular movement θ1 55 to the fiber. However, the spinning apparatus 60 is located at a distance from the heating apparatus 15 such that the fiber has cooled down somewhat by the time the coating is applied and further cooled once it comes into contact with the spinning apparatus. Thus, while the spinning apparatus imparts an angular movement θ1 55 at a lower point, a different angular movement, θ2 28, is imparted near the neck-down region 20 with the value of θ1<θ2 This is due in part to:                1. the long span of fiber between the neck-down region and the spinning apparatus;        2. the viscous drag characteristics of the coating;        3. the viscous drag characteristic of the neck-down itself;        4. slippage of the fiber in spinning apparatus; and        5. temperature differential of the fiber along its length.        
Consequently, the spin actually introduced compared to the spin attempted to be introduced is less than 100% but closely correlates with the spin attempted to be introduced.
In addition to “spin”, another metric regarding optical fibers that is measured during the manufacture is “twist”. The spin and twist metrics are related, and the terms are sometimes used interchangeably in the prior art. Sometimes, in the prior art there is no distinction between these words, or the differences are based on context, which may create confusion. As used herein, “spin” refers to the rotation introduced into the optical fiber in the molten state (i.e., prior to cooling), whereas “twist” refers to rotation introduced onto the optical fiber after it has cooled. Spin is imparted into the molten fiber, and is permanently fixed when the fiber has cooled. Twist refers to the mechanical rotational force imposed on the optical fiber after it has cooled and can be altered. Twist in the optical fiber typically occurs due to the spin process and, unlike spin, twist typically introduces torsional stresses on the fiber because it is introduced after the fiber has cooled and becomes relatively inelastic compared to its molten state. Twist can be “undone” or reduced by applying a rotational force in the other direction, whereas spin is permanent. Extreme amounts of twist can cause microscopic cracks, and contribute or cause the ultimate physical destruction of the fiber. Consequently, it is desirable to reduce or eliminate twist introduced on the fiber.
Hart recognizes that twist can occur and identifies one method of reducing twist on a optical fiber. Hart discloses “respooling” the fiber by unwinding it and rewinding the fiber, but this method of correcting twist is time consuming and labor intensive. It would be preferable to avoid or minimize the introduction of twist to acceptable levels during manufacturing and avoid additional handling or processing to reduce twist after manufacturing. Hart also discloses the use of a pure sinusoid spin function resulting in substantially equal and opposite twists being introduced onto the fiber for a given cycle, producing a net twist of zero. However, Henderson also discloses a variable spin function that is more effective in reducing PMD than a purely sinuisoidal spin function, but does not address methods for minimizing twist. The selection of one spin function (Hart) minimizes twist, while selection of the other function (Henderson) minimizes PMD.
Therefore, there is a need for a spin function that minimizes PMD while at the same time minimizes twist introduced into the fiber.